Implantable medical devices are devices that are implanted within the body of a patient so as to provide therapy to an organ of the patient. These types of devices have become increasingly common for the treatment of a variety of different medical ailments. For example, implantable cardiac stimulation devices, such as pacemakers and implantable cardioverter defibrillators (ICD's), have been in use for many years and provide electrical stimuli to the heart of the patient to regulate heart function. Similarly, neural stimulators are also now being implanted into patients' bodies in order to provide electrical stimulation to selected regions of the patient's brain to regulate brain function. Other medical devices can include devices for stimulating other organs or tissues, such as the pancreas, kidneys, etc.
Typically, an implantable medical device is equipped with a battery that supplies low voltage DC power to the implanted device. Generally, known low voltage batteries such as lithium, iodine, silver vanadium oxide (SVO) or lithium monofluoride batteries are used to provide power to the implantable device. Typically, these batteries provide output voltages in the range of 2 to 4 volts DC. However, it common for the implantable medical device to provide comparatively high voltage waveforms to an organ of the patient in order to regulate the organ's function.
For example, ICD's will often provide waveforms having peak voltage of approximately 800 volts or greater to the heart of a patient in order to terminate ventricular fibrillation. Similarly, neural stimulators will provide voltages in the range of 20 to 50 volts in order to terminate epileptic episodes. Hence, many implantable medical devices are equipped with a high voltage converter that uses the relatively low output voltage provided by the battery to develop a high voltage output signal that can be applied to one or more high voltage capacitors. When the capacitor(s) voltage reaches a programmed value, the charge stops and next the capacitor(s) voltage is applied to the organ of the patient using specialized HV switching means.
A typical high voltage converter used in implantable medical devices generally has a primary winding of a transformer being connected to the positive and negative plates of a battery via a switch, such as a transistor. One or more bypass capacitors are connected in parallel to the battery such that the bypass capacitors help maintain the voltage supplied to the transformer primary at a stable level during charging. A high voltage rectification circuit that includes one or more delivery capacitors is typically connected to the secondary winding of the transformer. Moreover, a switching network is generally attached to the switch such that the switch can be toggled at relatively high frequency. When the switch is turned on, an increasing current is provided to the primary winding of the transformer which stores energy as magnetic flux in the core of the transformer. When the switch is turned off, the stored energy is transferred from the core to the load via the secondary winding. Hence, by repeatedly cycling the switch at a relatively high frequency, energy can be transferred from the battery to the high voltage capacitors.
To improve the efficiency of this high voltage charging process, the bypass capacitors that are used are typically capacitors that have particularly low equivalent series resistance. Lower equivalent series resistance among the capacitors results in less of the battery's energy being dissipated in the form of heat in the resistance. Of course, in implanted medical devices, conservation of battery power is important as replacement of the battery often requires an invasive surgical procedure.
One common family of capacitors that is used as bypass capacitors in high voltage converter circuits for implantable medical devices is ceramic dielectric capacitors with a Y5V dielectric. Advantageously, ceramic capacitors are known to have particularly low equivalent series resistance and, thus, increases the efficiency of converter circuits.
One problem with ceramic capacitors is that these types of capacitors are typically not self-healing. In other words, if cracks develop in the ceramic dielectric, leakage currents will then flow from one plate of the capacitor to the other. This leakage current also represents an unwanted dissipation of the battery's energy and discharge it in a relatively short period. Typically, with the electrolytic capacitors that are also commonly used in implantable medical devices, leakage paths that develop between the plates as a result of applied voltages are often repaired by the electrolyte. Consequently, electrolytic capacitors have reduced leakage currents after an internal defect but, unfortunately, they also have a higher equivalent series resistance.
Hence, while ceramic capacitors are generally preferred to be used as the bypass capacitors in high voltage converter circuits for implantable medical devices, these ceramic capacitors also may produce leakage paths through which battery power can be dissipated when the bypass capacitors are not being charged. Again, dissipation of battery power is undesirable due to the difficulties associated with replacing batteries in implantable medical devices and also the consequences of battery failure in an implanted device. Hence, there is a need for a high voltage converter circuit for an implantable medical device that permits efficient charging of the HV capacitor but also reduces unwanted dissipation of battery power during the quiescent period of the high voltage converter circuit in an event of capacitors degradation. To this end, there is a need for a high voltage converter circuit for an implantable medical device with which high efficiency capacitors, such as ceramic capacitors, can be used as bypass capacitors while the effect of such leakage current that may occur with such types of capacitors can be reduced or eliminated.